Expansible shell mold with refractory slip cover and the method of making same

ABSTRACT

A refractory shell mold for precision metal casting made by a &#39;&#39;&#39;&#39;lost-wax&#39;&#39;&#39;&#39; process and having a core anchored in the shell by attaching portions, at least one of which portions is covered by a small molded refractory slip cover. Such slip cover may be placed on the core in the wax pattern die before the pattern is formed. It is formed to close tolerances to position the core with a degree of accuracy not attained by previously known methods.

[4 1 Mar. 27, 1973 [54] EXPANSIBLE SHELL MOLD WITH REFRACTORY SLIP COVER AND THE METHOD OF MAKING SAME [75] Inventor:

m t e s e n O .H L

John M. Webb, Cl l d Oh Primary Examiner-J. Spencer Overholser Assistant ExaminerJ. E. Roethel [73] Assignee: Edward J. Mellen, Jr., East Cleveland, Ohio AttorneyMcCoy, Greene & Howell [57] ABSTRACT A refractory shell mold for precision metal casting made by a lost-wax" process and having a core Appl. No.: 135,557

52 US. Cl. ......................164/366, 164/25, 164/30, anchored in the shell by attaching portions, at least one of which portions is covered by a small molded 164/397 ..B22c 9/10 164/9,10,1l, 23, 24, 25,

[51] Int. refractory slip cover. Such slip cover may be placed on the core in the wax pattern die before the pattern is [58] Field of Search............

formed. It is formed to close tolerances to position the core with a degree of accuracy not attained by previously known methods.

15 Claims, 7 Drawing Figures References Cited UNITED STATES PATENTS 2,756,475 Hanink et al. 1 64/30 EXPANSIBLE SHELL MOLD WITH REFRACTORY SLIP COVER AND THE METHOD OF MAKING SAME The present invention relates to shell molds for precision metal castings having cores which are accurately positioned to maintain close tolerances and more particularly to shell molds having small refractory slip covers mounted on attaching portions of the core.

Heretofore, it has been customary to provide a ceramic core with exposed attaching portions at opposite sides or opposite ends of the core which pro jected through the wax pattern and fitted in the shell mold to hold the core in position after the wax was removed. A thin layer of wax was spread manually over one of such exposed portions to prevent it from becoming bonded to the outer shell and to provide a slip joint after wax removal. However, it was difficult to apply the wax layer evenly and to provide the complete coverage needed. Because the wax layer resulting from such manual application usually had a substantial thickness (for example, 0.004 inch or greater), accurate positioning of the core in the shell mold was not obtained and extremely close tolerances in the final casting could not be maintained.

The present invention overcomes the difficulties described above and eliminates the costly manual wax application while providing a precision shell mold wherein the position of the core is maintained within close tolerances. The problem is solved by providing a small refractory slip cover of a size to fit one of the exposed attaching portions of the core and to fit against the wax pattern or other destructible pattern. Such slip cover may be placed on the core in the wax pattern die prior to forming of the wax pattern.

The recess in the slip cover is preferably the same size and shape as the attaching portion of the core and the cross-sectional width of the recess at any plane is preferably about equal to the corresponding cross-sectional width of the attaching portion (for example, no more than about 0.002 inch greater than that of the attaching portion). The process of this invention makes it possible to locate the core precisely within the shell mold and to provide close tolerances which were not attained when using previously known methods.

An object of the present invention is to provide a simple, reliable shell molding process for consistently producing hollow metal castings within close tolerance ranges. j

A further object of the invention is to provide simple, inexpensive means for accurately locating a refractory core in a shell mold.

Another object of the invention is to eliminate manual waxing of core prints".

A still further object of the invention is to provide more reliable expansible means for supporting a refractory core in a shell mold.

Another object of the invention is to reduce scrap loss in the manufacture of refractory cores.

These and other objects, uses and advantages of the invention will become apparent to those skilled in the art from the following drawings, description and claims.

In the drawings:

FIG. 1 is a foreshortened top view with portions broken away and shown in section illustrating a wax pattern and refractory core having refractory slip covers of the present invention mounted on attaching portions of the core, the outline of the shell mold being shown in dot-dash lines;

FIG. 2 is a transverse sectional view taken on the line 2-2 of FIG. 1, the outline of the shell mold being shown in dot-dash lines;

FIG. 3 is a transverse sectional view taken on the line 33 of FIG. 1 and on the same scale;

FIG. 4 is a foreshortened perspective view of one of the refractory slip covers of FIG. 1 on a larger scale;

FIG. 5 is a perspective view of the other refractory slip cover of FIG. 1;

FIG. 6 is a transverse sectional view of a wax pattern die taken on the line 6-6 of FIG. 7; and

FIG. 7 is a foreshortened sectional view taken on the line 77 of FIG. 6 with parts broken away and shown in section.

Referring more particularly to the drawings, FIGS. 1 and 2 show a conventional wax pattern A covering a conventional refractory core C. The wax pattern is in the form of a hollow turbine blade whose walls gradually decrease in thickness from the leading edge to the trailing edge.

The refractory core C has a curved body portion 2 which is shaped generally like a turbine blade and has straight leading and trailing edges 3 and 4 and flat upper and lower end edge portions 5 and 6 with flat surfaces located in parallel planes at opposite ends of said body portion 2. The core C also has upper and lower exposed attaching portions 7 and 8 which project outwardly beyond the edge portions 5 and 6, respectively, so as to project beyond the wax pattern A into the shell mold B which is formed over the wax pattern. The core C also has a narrow exposed trailing edge portion 9 at the trailing edge 4 of the core which also projects outwardly beyond the wax pattern into the shell mold B. The trailing portion 9 usually has a uniform width and substantially uniform cross section and is relatively straight as shown in FIG. 1.

It will be understood that refractory cores similar to the core C are well known in the art and that the use of attaching portions, such as the portions 7, 8 and 9, is conventional. These attaching portions are often referred to on plan views of refractory cores as core prints because they project outwardly beyond the wax pattern. Because the refractory core usually has a coefficient of expansion different from that of the shell mold, it is necessary to provide for expansion at some of the attaching portions. This is usually taken care of by manually painting or spreading a thin layer of wax over one or more of the attaching portions after the wax pattern has been molded around the core. One of the attaching portions is not coated with wax so that it will become bonded or fixed to the refractory material of the shell mold. The other attaching portion or portions is painted with wax as explained above to provide a wax coating which is very thin and may have a thickness of less than 0.030 inch. This coating must be thin so that, after the wax is burned out, the clearance between the attaching portion and the shell mold is minimized. This is necessary to enable the attaching portion to maintain the refractory core approximately in the proper position during casting of the metal in the shell mold.

The process of the present invention eliminates the need for applying wax coatings to the attaching portions and makes it possible to reduce scrap loss in the manufacture of the shell mold while at the same time providing more accurate positioning of the core in the mold. This is accomplished by providing one or more refractory slip covers for the projecting attaching portions of the core. Ordinarily all but one of such attaching portions is provided with a slip cover. In the case of simple cores which have only two attaching portions, one refractory slip cover is sufiicient. FIGS. 1 and 2 illustrate a more complicated refractory core having three attaching portions 7, 8 and 9. The attaching portion 7 shown herein has a generally oval cross section and a thickness substantially equal to that of the leading portion 11 of the core and is located at the side of the core remote from the trailing portion 12. The portion 7 has no slip cover so that it becomes anchored in the shell mold B.

As herein shown the lower exposed attaching portion 8 is in the form of a thin tab which is flat and has a uniform rectangular cross section as shown in FIG. 3. The portion 8 is also located at the leading portion 11 of the core.

The exposed trailing edge portion 9 is provided to support the trailing portion 12 of the core in the shell mold. In the practice of the present invention such portion is completely covered by the narrow refractory slip cover 10 shown in FIG. 4 which butts against the straight trailing edges 15 of the wax pattern.

As herein shown the wax pattern A has conventional leading and trailing portions 13 and 14 which conform to the leading and trailing portions 11 and 12 of the refractory core C. These wax portions gradually decrease in thickness from the leading edge 3 toward the trailing edge and terminate in straight trailing edges 15, which may be parallel to the straight trailing edge 4 of the core. The wax pattern also has flat end faces 16 and 17 located in parallel planes flush with the surfaces of the core portions and 6. The wax pattern terminates at the surfaces 15, 16 and 17, the attaching portions 7, 8 and 9 extending outwardly beyond such surfaces so as to extend beyond the wax pattern into the shell mold to provide supports for the core.

As shown in FIG. 4 the refractory slip cover has a uniform U-shaped cross section throughout its length and has a straight edge portion 18 of curved cross section and flat narrow side portions 19 of uniform width integral with the portion 18. The two side portions 19 have straight side edges 21, which are located in the same plane for engagement with the straight trailing edges of the wax pattern. The slip cover 10 also has straight end edges 22 at its opposite ends, which are located in parallel planes containing the flat surfaces 16 and 17, respectively, of the wax pattern. The interior surfaces of the slip cover 10 are shaped to conform to the exterior surfaces of the exposed trailing edge portion 9 of the core so that the slip cover fits snugly on the core as shown in FIG. 2 while at the same time providing a slip joint to accommodate expansion.

FIG. 5 illustrates a refractory slip cover for covering the lower attaching portion 8 of the core. The cover 20 is preferably molded with thin walls of substantially uniform thickness. As herein shown the cover 20 is molded in the form of a rectangular box having flat rectangular side walls 24 for engaging the flat side faces of the attaching portion 8 and flat parallel end walls 25 for engaging the flat side edges of the portion 8. The slip cover has a narrow flat bottom wall 26 which engages the end of the portion 8 and has a flat edge face 27 located in a plane parallel to the wall 26. The face 27 and the peripheral edge of the slip cover 20 preferably engage the lower end face 17 of the wax pattern around the periphery of the attaching portion 8 as indicated in FIG. 1. The shape of the slip cover 20 may vary considerably, but it is preferable to provide such slip cover with an internal cavity of essentially the same size and shape as the attaching portion 8 so that it fits snugly on the attaching portion and completely covers it with little or no clearance at the surface 27.

In carrying out the process of the present invention the refractory core C is molded or otherwise formed in the conventional manner and such core is then placed in a convention wax pattern die. The wax pattern A is then formed in the conventional manner in such die, for example to the shape shown in FIGS. 1 and 2 so that the attaching portions 7, 8 and 9 are exposed and extend outwardly beyond the wax pattern. After the wax pattern is removed from the wax pattern die, the slip cover 10 and 20 are slid into place on the attaching portions 9 and 8, respectively, as shown in FIG. 1. Such slip covers 10 and 20 may also be applied to the core prior to forming'the wax pattern if the wax pattern die is provided with special recesses to receive such slip covers (as, for example, in the embodiment of FIGS. 6 and 7).

After the slip covers 10 and 20 have been mounted on the core, the shell mold B is formed around the wax pattern and the core in any suitable manner. The slip covers are adhered or held in place on the attaching portions 8 and 9 so that they will remain in the proper position during forming ofthe shell mold. When the refractory slip covers are properly formed to fit snugly on the attaching portions, there is no need for special means to adhere or hold the slip covers in place because the frictional force is adequate to maintain them in place.

The mold B may be formed in a die or mold or by a pressing operation but is preferably formed by repeatedly dipping the wax pattern and core in a ceramic slurry and dusting the outer ceramic layer as disclosed, for example,'in US. Pat.. No. 2,932,864 or US. Pat. No. 2,961,751. After a series of layers of refractory material are built up over the wax pattern by the dipping, dusting and drying operations, the resulting shell mold B has an exterior configuration which conforms generally to that of the exterior surfaces of the wax pattern and the die M is g portions 7, 8 and 9. The shell mold may be generally as illustrated, in FIGS. 1 and 2 and may have leading and trailing portions 28 and 29 and upper and lower end portions 31 and 32. It will be seen from these FIGS. that the attaching portion 7 is anchored in the upper end portion 31 of the shell mold, that the refractory slip cover 10 is anchored in the trailing portion 29 of the mold, and that the slip cover 20 is anchored in the lower end portion 32. However, the attaching portions 8 and 9 can move slightly relative to the slip covers to accommodate expansion or contraction of the core.

The slip covers and thus prevent damage to the core during casting of the metal and at the same time provide accurate positioning of the core relative to the shell mold B. Because the slip covers can be formed with very close tolerances, it is possible to produce cast metal articles to close tolerances which could never be provided by the prior art methods.

When conventional wax pattern dies are employed, which have no special provision for the slip covers of this invention, the slip covers are applied after the wax pattern and core are removed from the wax pattern die. If desired the wax pattern die can be provided with a cavity or recess of a size to receive the slip cover of this invention. This is illustrated in the embodiment of FIG. 6 and 7. As shown in those Figures, the wax pattern die M is designed to form a wax pattern for a hollow. turbine blade. The refractory core C fits in the die M and 0 extends through the center of the mold cavity 37.

The die M is illustrated schematically, it being understood that such die may be of conventional construction except for the special recess for the refractory slip cover of this invention. As herein shown the die M has upper and lower parts 34 and 35 which fit together to define the mold cavity 37, and the upper part 34 has side flanges 36 to assist in locating the two die parts relative to each other. The two die parts fit together at the parting surface 28 of the lower die part 35 so as to close and seal the mold cavity throughout the periphery of the cavity in the conventional manner. As herein shown, the die cavity has straight outer side edges 39 and 40 at the parting surface 38 and has straight upper and lower edges 41 and 42 at the opposite ends of the mold cavity 37. The die cavity is curved and conforms generally to the exterior shape of the refractory core C, which is curved and has a leading portion 43 and a trailing portion 44 of reduced thickness.

Upper and lower exposed attaching portions 45 and 46 are provided at opposite ends of the refractory core and have flat outer faces 46 and 47, respectively. The attaching portion 45 and 46 extend beyond the edges 41 and 42, respectively, of the die cavity and fit in recesses of the die M in the conventional manner, but the recess for the lower attaching portion 46 is enlarged to receive the special refractory slip cover 30 of this invention. Thus the upper attaching portion 45 fits snugly in the die M at one end of the die cavity and the slip cover 30 fits snugly in the recess at the opposite end of the die cavity, with its flat bottom portion 49 in engagement with the flat surface 51 of the die M and with its curved side portion 50 in contact with the corresponding curved surface 52 of the die M throughout the periphery of the portion 46.

The attaching portions 45 and 46 of the core C may be relatively thick as shown herein or may be reduced in size. The thickness is usually at least 0.05 inch and is more often in the range of 0.1 to 0.3 inch in this type of arrangement. Of course, the attaching portions 8 and 9 of the core C-would usually be smaller than the said portion 46.

In carrying out the process of the present invention, the destructible pattern A, the refractory shell mold B and the refractory core C may be formed from conventional materials by conventional procedures. The pattern A may, for example, be formed of known destructible material such as frozen mercury, a mercury alloy, a watersoluble material, a destructible synthetic resin or plastic, or a wax as disclosed, for example, in the following U.S. Pat. Nos. 2,752,653; 3,063,113; 3,177,537; 3,208,115; 3,263,286; 3,426,834 and 3,465,808. The core C and each of the refractory slip covers 10, 20 and 30 may be made in the conventional manner using a conventional core composition. Such core composition may, for example, be of the type disclosed in U.S. Pat. No. 2,752,653 or U.S. Pat. No. 3,356,130. The techniques applied and the materials used to form the shell mold B may be of the conventional type and may be as disclosed in any of the following U.S. Pat. Nos. 2,806,271; 2,815,553; 2,932,864; 2,961,751; 3,005,244; 2,011,986; 3,296,666; 3,465,808 and 3,508,602.

The process used to form each of the slip covers 10, 20 and 30 is preferably similar to conventional processes for making small refractory cores and may include a conventional molding step and a conventional heating step. If desired, the slip cover can be fired at a temperature of 1,800 to 2,200 F. to provide improved strength before the slip cover is placed on the attaching portion of a core, but this may not be essential.

The materials used to form each of the slip covers 10, 20 and 30 are preferably such as to provide the slip cover with substantially the same coefficient as expansion as that of the shell mold B, and such materials may be the same as or essentially the same as those used in the shell mold. The refractory slip covers usually are formed using refractory particles held together by suitable binders, such as ethyl silicate, colloidal alumina, sodium silicate, colloidal zirconia, colloidal silica or the like. Both a high-temperature binder and a lowtemperature binder may be employed as in conventional core recipes. The refractory material used in the practice of this invention to form the slip covers, the cores and the shell molds may be any conventional ceramic material used for making refractory cores or shell molds, such as zirconite, fused silica, aluminum silicate, zirconium silicate, alumina or other refractory oxide, etc.

The slip covers of the present invention are preferably made by conventional methods used to make refractory cores including injection molding, transfer molding, casting or ceramic extrusion. However, they can also be formed like shell molds by repeated dipping and drying operations as disclosed, for example, in U.S. Pat. No. 2,932,864 or U.S. Pat. No. 3,508,602. If necessary, the internal surface could be ground with a diamond wheel to provide the desired accuracy.

The slip covers can also be formed from solid plates of quartz glass or other refractory material, for example, by cutting or burning a slot in a plate or by welding several plates together in parallel relation. it may be desirable to form the slip cover of glass or other material which provides a smooth surface and does not bond to the shell 'mold so as to permit relative axial movement between the cover and the shell mold, at least in those cases where the coefficient of expansion of the slip cover is different from that of the shell mold.

In order to facilitate relative movement between the slip cover and the associated attaching portion of the refractory core, the internal surface of the cover and the external surface of the associated attaching portion should substantially conform to a cylinder that is, a surface generated by a line moving parallel to a fixed line (and preferably parallel to the direction of expansion or contraction of the core relative to the shell mold). The terms cylinder and cylindrical are used in the present description and claims in the broad mathematical sense and is, of course, not restricted to cylinders of oval cross section.

While the size and shape of the slip covers of this inverition may vary, such covers preferably have thin walls. Thick walls should be avoided because they affect the heat transfer characteristics of the shell mold and make it more difficult to produce good metal castings. The slip covers should have walls with a thickness not in excess of 0.1 inch and preferably in the range of 0.01 to 0.06 inch.

The slip cover shown herein is mounted on an exposed rectangular trailing edge portion 9 of the turbine blade core C, which has a width of 0.1 to 0.5 inch, a

length of 2 to 10 inches, and a thickness of 0.01 to 0.1 inch (usually at least 0.03 inch). The slip cover 10 has the same width and length and has a thickness of 0.01 to 0.1 inch, preferably no more than twice the thickness of portion 9 and usually 0.02 inch or more to facilitate handling.

The internal cylindrical surface of the slip cover 10, or 30 and the external cylindrical surface of the associated attaching portion 8, 9 or 46 should be accurately formed with a minimum clearance so that the core will be accurately positioned relative to the shell mold during metal casting. In order to obtain a high degree of accuracy, the cross-sectional width of the cylindrical recess in the slip cover 10, 20 or 30 should be no more than 0.002 inch greater than the cross-sectional width or thickness of the associated attaching portion 9, 8 or 46. It is relatively simple to form the internal surfaces of the refractory slip covers of this invention to within one or two thousandths of an inch. The tolerance can be maintained at 0 to +0002 inch so that the variation in location of the core relative to the shell mold is no more than a few thousandths of an inch. Prior to this invention it was common for the variation in core location to be in the range of 0.005 to 0.010 inch. The present invention can improve the accuracy 50 to 100 percent or more and increase foundry efficiency for cored parts.

Prior to this invention, there have been instances where locating pins have been used to assist in positioning the core in special applications. For example, in connection with a directional solidification process for making ceramic molds, it has been proposed to place refractory locating pins in the wax patterns with their inner ends at the surface of the core and their outer end portions projecting radially outwardly so as to become embedded in the shell mold. Such locating pins have limited application, however, and are undesirable for ordinary casting operations. They do not provide a satisfactory solution to the problem of providing extreme accuracy while permitting the necessary relative expansion of the mold parts.

It will be understood that the above description is by way of illustration, rather than limitation, and that, in accordance with the provisions of the patent statutes, variations and modifications of the specific methods and products disclosed herein may be made without departing from the spirit of the invention.

Having described my invention, I claim:

1. In a process of making a shell mold comprising molding a destructible pattern of a predetermined shape around a refractory core having minor exposed attaching portions projecting outwardly through said pattern and thereafter forming a refractory shell of substantial thickness by repeatedly dipping the destructible pattern in a ceramic slurry and drying each successive ceramic layer so that at least one of said exposed attaching portions is anchored in the shell to maintain the position of the core when the, pattern is destroyed, the steps which comprise: forming a refractory slip cover with a predetermined shape to cover another of said exposed portions, and mounting said slip cover on such other exposed portion before completion of said shell so that such slip cover provides an expansible joint for accurately maintaining the position of the core relative to the shell after the pattern is destroyed and during metal casting.

2. A process as defined in claim 1 wherein said slip cover is formed of a refractory material having expansion characteristics generally the same as those of said shell to avoid cracking of said slip cover during expansion of said shell.

3. A process as defined in claim 2 wherein the peripheral edge of said slip cover is shaped to fit against said destructible pattern around the periphery of said other exposed portion of the core.

4. A process as defined inclaim 3 wherein said slip cover has walls with a thickness of about 0.01 to 0.06 inch.

5. A process as defined in claim 3 wherein said slip cover is mounted in a predetermined position on the attaching portion of the core and held in that position during molding of the destructible pattern.

6. A process as defined in claim 3 wherein said core has more than two of said exposed attaching portions, and two of said attaching portions are covered by refractory slip covers.

7. A process as defined in claim 1 wherein said pattern has the shape of a turbine blade and said other attaching portion comprises the thin trailing edge of a turbine blade core.

8. In a process comprising forming a refractory core with attaching portionsthereom placing the core in a pattern die having a mold cavity of a predetermined shape with said attaching portions projecting outwardly beyond the mold cavity and resting on recessed portions of the die outwardly of said cavity, and filling the mold cavity with a destructible material to form a destructible pattern around said core, the improvement which comprises placing a small refractory slip cover over one of said attaching portions, providing one of said recessed portions of the pattern die with a size and shape to fit and conform to the exterior surface of said slip cover, and holding the slip cover in a predetermined position during molding of said destructible pattern.

9. A refractory shell mold for precision metal casting comprising a multi-layer outer refractory shell with a mold cavity of a predetermined shape, a refractory core of a predetermined shape in said mold cavity having attaching portions projecting into said shell to hold the core accurately in position during metal casting, and a small refractory slip cover anchored in the refractory shell and covering one of said attaching portions.

10. A shell mold as defined in claim 9 wherein the recess in the refractory slip cover for receiving said last-named attaching portion has a cross section which is substantially the same as that of such attaching portion and a width which is no more than about 0.002 inch greater than the corresponding dimension of said attaching portion.

11. A shell mold as defined in claim 9 wherein said last-named attaching portion has a generally rectangular cross section and said slip cover has a wall thickness of about 0.01 to about 0.1 inch.

12. A shell mold as defined in claim 9 wherein said last-named attaching portion has a thickness of about 0.01 to about 0.1 inch and said slip cover has a wall thickness not in excess of about 0. 1 inch.

13. A shell mold as defined in claim 9 wherein said mold cavity has the shape of a hollow turbine blade and said last-named attaching portion is located at the trailing edge of the turbine blade core.

14. A shell mold as defined in claim 13 wherein said last-named attaching portion has a width of 0.1 to 0.5 inch, a length of 2 to 10 inches and a thickness of 0.01 to 0.1 inch, and said slip cover has a wall thickness not in excess of 0.1 inch and no more than twice the thickness of said last-named attaching portion.

15. A shell mold as defined in claim 14 wherein the recess in said slip cover has a cross-sectional width of no more than about 0.002 inch greater than that of the corresponding cross section of said last-named attaching portion. 

1. In a process of making a shell mold comprising molding a destructible pattern of a predetermined shape around a refractory core having minor exposed attaching portions projecting outwardly through said pattern and thereafter forming a refractory shell of substantial thickness by repeatedly dipping the destructible pattern in a ceramic slurry and drying each successive ceramic layer so that at least one of said exposed attaching portions is anchored in the shell to maintain the position of the core when the pattern is destroyed, the steps which comprise: forming a refractory slip cover with a predetermined shape to cover another of said exposed portions, and mounting said slip cover on such other exposed portion before completion of said shell so that such slip cover provides an expansible joint for accurately maintaining the position of the core relative to the shell after the pattern is destroyed and during metal casting.
 2. A process as defined in claim 1 wherein said slip cover is formed of a refractory material having expansion characteristics generally the same as those of said shell to avoid cracking of said slip cover during expansion of said shell.
 3. A process as defined in claim 2 wherein the peripheral edge of said slip cover is shaped to fit against said destructible pattern around the periphery of said other exposed portion of the core.
 4. A process as defined in claim 3 wherein said slip cover has walls with a thickness of about 0.01 to 0.06 inch.
 5. A process as defined in claim 3 wherein said slip cover is mounted in a predetermined position on the attaching portion of the core and held in that position during molding of the destructible pattern.
 6. A process as defined in claim 3 wherein said core has more than two of said exposed attaching portions, and two of said attaching portions are covered by refractory slip covers.
 7. A process as defined in claim 1 wherein said pattern has the shape of a turbine blade and said other attaching portion comprises the thin trailing edge of a turbine blade core.
 8. In a process comprising forming a refractory core with attaching portions thereon, placing the core in a pattern die having a mold cavity of a predetermined shape with said attaching portions projecting outwardly beyond the mold cavity and resting on recessed portions of the die outwardly of said cavity, and filling the mold cavity with a destructible material to form a destructible pattern around said core, the improvement which comprises placing a small refractory slip cover over one of said attaching portions, providing one of said recessed portions of the pattern die with a size and shape to fit and conform to the exterior surface of said slip cover, and holding the slip cover in a predetermined position during molding of said destructible pattern.
 9. A refractory shell mold for precision metal casting comprising a multi-layer outer refractory shell with a mold cavity of a predetermined shape, a refractory core of a predeterminEd shape in said mold cavity having attaching portions projecting into said shell to hold the core accurately in position during metal casting, and a small refractory slip cover anchored in the refractory shell and covering one of said attaching portions.
 10. A shell mold as defined in claim 9 wherein the recess in the refractory slip cover for receiving said last-named attaching portion has a cross section which is substantially the same as that of such attaching portion and a width which is no more than about 0.002 inch greater than the corresponding dimension of said attaching portion.
 11. A shell mold as defined in claim 9 wherein said last-named attaching portion has a generally rectangular cross section and said slip cover has a wall thickness of about 0.01 to about 0.1 inch.
 12. A shell mold as defined in claim 9 wherein said last-named attaching portion has a thickness of about 0.01 to about 0.1 inch and said slip cover has a wall thickness not in excess of about 0.1 inch.
 13. A shell mold as defined in claim 9 wherein said mold cavity has the shape of a hollow turbine blade and said last-named attaching portion is located at the trailing edge of the turbine blade core.
 14. A shell mold as defined in claim 13 wherein said last-named attaching portion has a width of 0.1 to 0.5 inch, a length of 2 to 10 inches and a thickness of 0.01 to 0.1 inch, and said slip cover has a wall thickness not in excess of 0.1 inch and no more than twice the thickness of said last-named attaching portion.
 15. A shell mold as defined in claim 14 wherein the recess in said slip cover has a cross-sectional width of no more than about 0.002 inch greater than that of the corresponding cross section of said last-named attaching portion. 